Pipetting devices or pipettes, respectively, are used for dosing liquids, together with pipette points. Pipette points are little pipes with an upper opening for joining them with a pipette, a lower opening for the passage of liquid and a passage channel between the upper opening and the lower opening. Pipettes have mostly a neck for putting on the upper opening, or an accommodation for putting in that end of the pipette point which has the upper opening. Further, they have a gas displacement equipment, which is realised in piston pipettes as a cylindrical displacement chamber with a piston movable therein. In manual pipettes, the piston is moved against the action of a pull-back spring by means of an actuation button. In electric pipettes, an electric driving motor is coupled to the piston via a gear, in order to move it to and from in the cylinder. The gas displacement equipment is connected to the upper opening of the pipette point, which is detachably held on the pipette, via a connection channel that runs through the neck or runs out in the accommodation.
By means of the gas displacement equipment, an air column or a column of another gas is moved in order to aspirate liquid into the pipette point and to eject it from the same. When the gas column is pushed away from the pipette point, a certain amount of liquid is aspirated into the passage channel of the pipette point via the lower opening when the pipette point is immersed into a liquid. By moving the gas column towards the pipette point, a certain amount of liquid is delivered from the passage channel through the lower opening. The dosing volume depends on the degree of movement of the gas column. The latter is set by the stroke of the piston in piston pipettes.
Pipette points are mostly replaced by fresh pipette points after dosing has taken place, in order to avoid contamination of subsequently pipetted liquids by remaining liquid. Pipette points for single use are mostly realised from plastic material.
In manual pipettes with adjustable dosing volume, a shiftable limit stop for delimiting the piston stroke is available. The shifting takes place by means of a little turning wheel, which acts on the limit stop via a gear, wherein the dosing volume that is set can be read by means of a metre.
In electronic pipettes it is known, e.g., to use step motors and to ensure the reaching of a dosing volume which is set by applying a corresponding number of control pulses. Furthermore known is counting the rotations and stopping the driving motor when a number of rotations corresponding to the dosing volume has been reached.
The known manual pipettes have the disadvantage to have only a slight stroke of the actuation button at small dosing volumina, which results in a poor precision or a poor control, respectively, in the delivery of the liquid. Manual and electronic pipettes have the disadvantage that the effort to overcome the frictions of the sealings between piston and cylinder requires an increased expenditure of energy. Further, the pistons, cylinders and sealings disposed there between have to be maintained or greased, respectively. Generally, the setting range of piston pipettes is small. In order to cover greater ranges, several piston pipettes with different piston cross sections have to be provided.
Departing from this, the present invention has the objective to provide a pipetting device which makes it possible to work with the same actuation stroke at all dosing volumes that are set. Further, the pipetting device should have an enlarged setting range of the dosing volumina. In addition, the gas displacement equipment should have less expenditure for maintenance.